The Limit of Aerial Evolution

December 11, 2011 at 7:20 pm

The thought of a Cessna 152 dropping out of the sky from 20,000 ft at 300 mph and pulling out of the plunge five feet off of the ground would scare most pilots to death. But for Argentavis magnificens, it was a lifestyle. Six million years ago, the twenty-three foot wide monster bird would come streaking out of the Andes en route to breakfast, lunch, and dinner.

Argentavis magnificens was the largest bird to take to the skies in Earth’s history before it died out, but it probably couldn’t fly on its own. Argentavis most likely was a soaring bird, taking wing over the uplift from the trade winds hitting the Andes mountains. While hunting over the papas of modern-day Argentina, the bird would have been entirely dependent on thermals for positive lift. With a takeoff mass of 154 lbs, it would have had to have flight muscles more than 200% larger than is estimated that it actually had based on fossil evidence.

Campbell et al assume that it had to have at least a running start plus a headwind to get off the ground—something that might not have been hard to come by. The trade winds coming in off the coast are very predictable, and the mountainous terrain that Argentavis would have called home would provide plenty of cliffs and slopes needed to get airborne. They calculate that with a 10 degree slope and an 11 mph headwind generated by running, the bird would have been able to get in the air.

Landing would be a whole separate question. Birds of this size would have had to be very careful not to damage a foot or wing when landing/crashing in a controlled manner. Because it couldn’t support its weight by flapping, landing an Argentavis would have been akin to landing a Space Shuttle without computers. The bird would essentially get one shot to come in at a shallow enough glide angle that it could pitch up to stall its wings, bleed off its forward velocity, and plop onto the ground.

While scientists are sure that Argentavis flew because of it’s bone densities and muscle cavities, they’re not sure how it hunted. It could have dive-bombed prey on the ground spotted from high altitude. However, Argentavis had one major flaw. Because it couldn’t climb by flapping its wings, it had to maintain enough momentum to climb out of the dive and make the next thermal. If it doesn’t make it, it crashes into the ground where it might not be able to take off again anytime soon. Considering that the Peregrine Falcon can reach velocities in excess of 240 mph while in a dive, it’s incredible what Argentavis might have been capable of.

Using the lift equation and assuming that the wingspan of the bird was 23 ft with a chord length of 4.5 feet, Argentavis was capable of cruising at a velocity of 150 mph in level flight. By comparison, the Peregrine Falcon has a cruising velocity of less than 60 mph. Undoubtedly, the falcon would be proportionally faster in a dive thanks to its exceptional aerodynamics, but velocities in excess of 300 mph for Argentavis are entirely possible.

The talon-eye coordination for these magnificent birds must have been incredible to observe as they would grab a meal from thousands of feet in the air at hundreds of miles per hour with zero margin for error. While aerodynamically they had enough maneuverability to do this, they were more likely to have scavenged for meals like the Condor of today does. It’s impressive size would have meant that it could consume most of a dead animal in one sitting, spacing out the amount of times that it would have to eat in a given week or month.

Truly evolution has produced incredible creatures capable of incredible feats. However ultimately, the lifestyle that this bird lived was too demanding of it. Perhaps it simply wasn’t sustainable, perhaps the wind currents that defined it’s habitat shifted, or perhaps the birds simply evolved into smaller species. Regardless, Argentavis demands respect as the greatest bird ever to take to the skies of Earth.

2 Comments

Great post!
I chuckled at your “talon-eye coordination” quip. Oh, and it’s “pampas” in Argentina, not “papas” (which means potato).
As for the feeding habits of the Argentavis, I imagine that it would have been much safer for them to hunt in the air, i.e., eat other flyers and hunt in mid-air!